This disclosure generally relates to electronic registration and, more particularly, to a system to improve image registration by electronic compensation of raster output scanner beam scan trajectory distortions.
Electrophotographic marking is a well-known and commonly used method of copying or printing documents. In general, electrophotographic marking employs a charge-retentive, photosensitive surface, known as a photoreceptor, that is initially charged uniformly. In an exposure step, a light image representation of a desired output focused on the photoreceptor discharges specific areas of the surface to create a latent image. In a development step, toner particles are applied to the latent image, forming a toner or developed image. This developed image on the photoreceptor is then transferred to a print sheet on which the desired print or copy is fixed.
The electrophotographic marking process outlined above can be used to produce color as well as black and white (monochrome) images. Generally, color images are produced by repeating the electrophotographic marking process to print two or more different image layers or color image separations in superimposed registration on a single print sheet. This process may be accomplished by using a single exposure device, e.g. a raster output scanner (ROS), where each subsequent image layer is formed on a subsequent pass of the photoreceptor (multiple pass) or by employing multiple exposure devices, each writing a different image layer, during a single revolution of the photoreceptor (single pass). While multiple pass systems require less hardware and are generally easier to implement than single pass systems, single pass systems provide much greater print speeds.
In generating color images, the ability to achieve precise registration of the image layers is necessary to obtain printed image structures that are free of undesirable color fringes and other registration errors. Precise registration of image layers in a single pass machine requires precise registration from one ROS to the next. One major cause of misregistration in multiple ROS systems is the differences in the beam scan trajectory of each ROS in the imaging system.
In general, a conventional ROS repeatedly scans a data modulated light beam over a photoreceptor surface in accordance with a predetermined raster scanning pattern to generate an image. Typically, a conventional ROS includes a laser diode or similar device to generate a light beam that is modulated in response to received data. The ROS further includes a rotating polygonal mirror block to repeatedly scan the light beam across the photoreceptor. As the photoreceptor is advanced in a process direction, the ROS repeatedly scans the modulated light beam across the surface of the photoreceptor in a fastscan direction that is orthogonal to the process direction.
Ideally, each scan of the light beam across the photoreceptor (generally identified herein as a beam scan) traces a straight line across the surface of the photoreceptor that is substantially normal to the movement of the photoreceptor in the process direction. Typically, each ROS introduces different pixel positioning errors that distort its beam scan. Thus, in a machine with more than one ROS, each ROS will likely have a different beam scan-trajectory. However, alignment of the ROS to the photoreceptor and non ideal optical components can curve the scan line in shapes, such as parabolic, which contributes to process-direction distortion (slow scan). Furthermore, variations in the angular speed of the rotating polygonal mirror can contribute to distortion in the cross-process direction (fast scan). This disclosure, and the exemplary embodiments described herein, are primarily directed to compensating for distortion in the process direction.
To achieve the color to color registration necessary to generate color images that are free of undesirable registration errors, the beam scan trajectory of each ROS must be within a relatively tight bound such that all scan trajectories are within a 50 micron envelope. Such tight registration tolerances are very difficult and very expensive to achieve solely by opto-mechanical means within the ROS. Systems for compensation and/or correction of beam scan distortions to improve registration errors have been proposed. However, many of these proposed systems correct only one type of distortion and often are themselves complex and expensive to implement. This disclosure also applies to controlling image-to-paper registration. Image-to-paper registration issues can occur in several possible scenarios. One concerns side 1 and side 2 registration in duplex printing. For example, the paper in one or both of the printing steps can be skewed in some predictable way. If this skew is not compensated for, any marks attempted to be printed at the same positions of both side 1 and side 2 will be unsuccessful. Another scenario relates to the relative placement of a marked image relative to the paper borders. Any misalignment of the marked image resulting from uncompensated paper skew will manifest itself as an improperly aligned image, relative to the paper borders. The presently disclosed embodiments can be used to skew the image in a like manner so that its position on the paper is not skewed, and the position of side 1 and side 2 are not skewed relative to each other for duplex printing.
The following references may be found relevant to the present disclosure and are hereby incorporated by reference in their entirety.
U.S. Pat. No. 5,430,472 to Curry discloses a method and apparatus for eliminating misregistration and bowing by controlling a composite light intensity profile and phase shifting of a spatial location at which the light intensity profile crosses a xerographic threshold in a two dimensional high addressability printer operating in an overscan mode.
U.S. Pat. No. 5,732,162 to Curry discloses a system for correcting registration errors in a printer with subscan precision. The system includes a memory device for storing sequential rasters of image data and an interpolator coupled to the memory device. The interpolator uses the rasters of image data from the memory device in conjunction with multiplication factors to calculate an interpolated resample value.